The invention relates generally to optical detection of chemical species, and more particularly to optical detection of chemical species by refractive index.
A particular problem of great concern in the aviation industry is the ability to detect aircraft icing. In recent years there have been a number of airplane crashes attributed to icing problems.
Optical detection, based on the unique optical properties of materials, is often highly advantageous. In particular optical detection schemes based on refractive index are often useful. There are several technologies for ice detection based on refractive index.
U.S. Pat. No. 4,851,817 issued Jul. 25, 1989 to Brossia et al describes a system for detecting water and icing on surfaces wherein changes in transmission of light through an optical fiber are produced by exposing a portion of the fiber to ambient conditions. The difference of refractive indexes between air, water and ice which contact the exposed portion changes the transmission properties of the fiber.
U.S. Pat. No. 4,913,519 issued Apr. 3, 1990 to Klainer et al describes an ice sensor based on differences in refractive index. A fiber optic or other waveguide is formed with spaced stripes of a clad material having an index close to ice with the core having an index less than water. When ice fills the gaps between the clad stripes, light is totally internally reflected by the clad, but when water or other materials having a greater refractive index fill the gaps, light is lost through the gaps and transmission through the fiber or waveguide is diminished.
U.S. Pat. No. 5,014,042 issued May 7, 1991 to Michoud et al describes a system in which light is transmitted through an optical channel having a prism with two plane faces at 90.degree. to each other forming an interface with the external environment. Light is incident at 45.degree. to one of the faces. If the external environment is air, the light is totally internally reflected towards the other face and back through the channel to a receiver. If water or ice is present at the interface, the light is refracted out of the prism, reducing or eliminating light transmission through the channel. The prism extends 7 cm out of the fuselage at the nose so that air flow causes rain to modulate the light transmission.
However the prior art for ice detection has many deficiencies. For example, the method of Klainer is vulnerable to contamination that can irreversibly fill the grooves allowing ice to form outside the sensing region and lead to a false negative reading. Michoud can only distinguish ice from water when the aircraft is moving. In addition, none of the systems can easily identify different substances of different refractive indexes. Consequently, a better ice detection system is sought.